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NATIONAL  RESEARCH 
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THE  LARGER  OPPORTUNITIES  FOR  RESEARCH 

ON  THE  RELATIONS  OF  SOLAR  AND 

TERRESTRIAL  RADIATION 


By  C.  G.  Abbot 

Smithsonian  Astrophysical  Observatory 


Published  in  the  Proceedings  of  the  National  Academy  of  Sciences, 
February,  1920,  vol.  6,  no.  2,  pages  82-95 


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NUMBER  7 


THE    LARGER    OPPORTUNITIES    FOR    RESEARCH    ON  THE 
RELATIONS  OF  SOLAR  AND  TERRESTRIAL  RADIATION* 

BY  C.  G.  ABBOT 
SMITHSONIAN  ASTROPHYSICAI,  OBSERVATORY 

The  earth  is  maintained  at  its  present  temperature  by  a  balance  be- 
tween the  solar  radiation  received  and  the  terrestrial  radiation  emitted. 
The  mean  intensity  of  the  solar  radiation  as  it  is  at  the  earth's  mean 
distance  outside  the  atmosphere  is  believed  to  be  well  known.  However, 
it  appears  to  be  variable  from  year  to  year  and  also  irregular  over  short 
intervals  of  days,  weeks  or  months.  These  variations,  which  have  been 
discovered  and  to  a  certain  extent  followed  for  about  fifteen  years  by  the 
Smithsonian  Astrophysical  Observatory  at  its  station  on  Mt.  Wilson, 
have  lately  been  compared  with  terrestrial  temperatures  and  atmospheric 
pressures  at  different  stations  of  the  world  by  several  meteorologists, 
notably  by  Clayton,  and  it  appears  that  the  effects  of  solar  variations  are 
important  as  governing  changes  of  terrestrial  temperature. 

This  indicates  two  highly  desirable  researches:  First,  to  make  daily 
observations  of  the  sun  competent  to  determine  its  variations.  Second, 
to  determine  by  further  statistical  inquiries  the  exact  influence  of  the 
variations  of  the  sun  on  the  terrestrial  phenomena. 

In  order  to  carry  out  the  first  research  thus  outlined,  it  requires  not 
less  than  four  special  solar  radiation  observing  stations  in  the  four  most 
cloudless  regions  of  the  earth.  One  such  is  now  in  existence  at  Calama, 
Chile,  where  it  is  being  maintained  by  the  private  funds  of  the  Smith- 
sonian Institution.  If  there  was  available  the  necessary  income  of  thirty 
or  forty  thousand  dollars  a  year,  the  Institution  would  establish  the  re- 
quired observing  stations  and  would  arrange  for  the  daily  wireless  trans- 
mission of  the  results  obtained  on  solar  radiation  so  that  they  might  be 
available  for  meteorologists  in  all  parts  of  the  world  to  use  for  forecasting 
purposes.  The  same  could  be  done  by  international  cooperation  if  due 
regard  were  given  to  making  the  measurements  homogeneous.  It  is  not 
probable  that  any  steps  of  this  kind  will  be  taken  within  a  year  or  two. 
By  that  time  the  series  of  measurements  being  made  at  Calama,  Chile, 

*  This  is  issued  as  the  second  of  a  series  of  research  surveys  prepared  under  the 
auspices  of  the  National  Research  Council. 

Published  also  in  the  Proceedings  of  the  National  Academy  of  Sciences,  6,  1920  (2). 

1 


2  ASTRONOMY:  C.  G.  ABBOT 

will  enable  meteorologists  to  determine  to  what  extent  the  results  will 
be  valuable  for  forecasting  purposes. 

It  would  promote  the  proposed  solar  investigation  if  there  could  be 
devised  autographic  recording  instruments  for  determining  the  exact 
value  of  the  cloudiness  at  all  times  of  the  day  when  the  sun  is  above  the 
horizon.  Hitherto  measurements  of  cloudiness  have  been  made  at  com- 
paratively few  stations,  and  as  they  depend  upon  personal  estimates  of 
observers  who  are  apt  to  overlook  wisps  of  cirrus  clouds  especially  dan- 
gerous for  solar  radiation  work,  and  as  they  are  usually  taken  only  two 
or  three  times  in  the  day,  they  are  quite  inadequate  to  enable  the  investi- 
gator interested  in  establishing  solar  radiation  stations  to  select  the  regions 
of  the  earth  best  meeting  his  requirements  of  cloudlessness.  It  would, 
therefore,  be  a  very  valuable  investigation  if  some  suitable  autographic 
method  of  measuring  cloudiness  could  be  devised  simple  enough  to  be  em- 
ployed at  numerous  stations  reported  to  be  cloudless. 

The  whole  subject  of  the  cloudiness  of  the  earth  is  very  important. 
Recent  measurements  of  Aldrich  show  that  a  cloud  surface  reflects  about 
78%  of  the  solar  radiation  incident  upon  it.  As  the  cloudiness  of  the  earth 
is  reputed  to  be  about  50%,  it  is  obvious  what  a  large  part  clouds  play  in 
determining  the  temperature.  They  are  effective  not  only  in  cutting  off 
incoming  radiation  from  the  sun  but  also  outgoing  radiation  from  the  earth. 
If  the  simple  autographic  apparatus  above  mentioned  were  available,  it 
should  be  employed  at  numerous  stations  in  all  parts  of  the  world  and  if 
possible  on  the  ocean. 

The  measurement  of  solar  radiation  at  several  stations  on  the  earth's 
surface  by  means  of  the  pyrheliometer  has  occupied  observers  in  many 
countries.  Several  series  of  observations  have  been  carried  on  for 
many  years  by  the  U.  S.  Weather  Bureau,  and  are  published  in  excellent 
form  in  the  Monthly  Weather  Review.  Many  copies  of  the  Angstrom 
pyrheliometer  and  of  the  Smithsonian  silver  disc  pyrheliometer,  whose 
constants  are  both  so  well  known  as  to  form  together  a  homogeneous  system 
of  pyrheliometry,  have  been  sent  out  to  many  quarters  of  the  world. 
Only  a  few  series  of  regular  observations  are  at  the  present  time  being 
conducted  with  them,  and  it  is  greatly  to  be  hoped  that  a  large  number 
of  such  series  may  be  conducted  regularly  in  the  future.  The  method  of 
reducing  the  results  and  publishing  them  can  hardly  be  improved  over 
that  which  is  followed  by  the  United  States  Weather  Bureau; -and  that 
may  well  be  taken  as  a  model. 

The  study  of  the  outgoing  radiation  from  the  earth  has  lately  been 
carried  on  by  several  observers,  notably  by  Mr.  Fowle,  who  has  investi- 
gated the  effects  of  water  vapor  in  long  atmospheric  columns  in  diminishing 
the  rays  of  long  wave-length  such  as  the  earth  sends  out.  His  investiga- 
tions extend  only  to  a  wave-length  of  17  microns  because  no  suitable  optical 
media  transparent  to  waves  of  longer  wave-length  are  available.  It  ap- 


ASTRONOMY:  C.  G.  ABBOT  3 

pears  that  potassium  iodide  may  be  such  a  medium  and  it  is  hoped  shortly 
to  produce  large  crystals  of  the  salt  suitable  for  making  prisms  for  the 
study  of  long  wave-lengths  of  terrestrial  radiation. 

In  the  meantime,  valuable  information  has  been  obtained  on  terrestrial 
and  atmospheric  long  wave-length  radiation  by  Dr.  A.  K.  Angstrom 
by  the  use  of  the  pyrgeometer  or  nocturnal  radiation  instrument  devised 
by  his  father.  An  instrument  applicable  for  the  same  purpose  has  also 
been  devised  by  Messrs.  Abbot  and  Aldrich  of  the  Smithsonian  Institution 
and  is  called  by  them  the  pyranometer.  Neither  instrument  is  entirely 
satisfactory  for  very  long  waves  since  both  depend  upon  a  knowledge  of 
the  absorption  of  blackened  surfaces  to  long  wave-lengths,  and  this  is  a 
matter  which  still  lies  in  doubt.  For  satisfactory  progress  in  this  line  of 
the  measurement  of  nocturnal  radiation,  a  new  instrument  depending 
upon  the  principle  of  the  hollow  chamber  or  absolutely  black  body  ought 
to  be  devised. 

The  study  of  the  effect  of  ozone  on  the  temperature  of  the  earth  is  one 
which  ought  not  to  be  neglected.  Next  to  cloudiness  and  atmospheric 
humidity,  ozone  is  thought  to  play  the  most  important  part  in  its  effect 
on  terrestrial  radiation;  first,  because  the  quantity  of  ozone  in  the  at- 
mosphere is  believed  to  be  variable,  and,  second,  because  its  effect  upon 
terrestrial  radiation  occurs  near  the  wave-length  10  microns  where  water 
vapor  is  almost  completely  transparent.  This  research  should  include 
the  determination  of  the  variations  of  the  quantity  of  atmospheric  ozone 
and  the  measurement  of  the  effect  of  its  changes  on  the  ozone  band  near 
10  microns  in  wave-length. 

A  fuller  discussion  of  the  several  points  which  have  been  raised  above 
will  be  found  in  the  following  papers. 

INVESTIGATIONS   IN   SOLAR  RADIATION 

Survey  of  the  Present  State  of  the  Field 

We  may  inquire:  First,  what  is  the  intensity  of  the  radiation  of  the 
sun  on  which  all  the  life  of  the  earth  depends;  second,  is  this  intensity 
constant  or  variable  from  day  to  day  and  from  year  to  year,  and  what 
are  the  effects  of  variability,  if  any? 

1.  The  Solar  Constant. — The  question  of  the  absolute  amount  of  the 
solar  radiation  interests  us  not  only  for  our  own  sake  but  for  the  sake  of 
the  future  generations.  What  a  valuable  thing  it  would  be  if  we  knew  at 
this  time  the  intensity  of  the  sun's  radiation  as  it  was  in  the  times  of  the 
Egyptians  and  the  Babylonians,  and  as  it  was  in  the  intervening  periods 
of  Rome  and  the  Middle  Ages.  It  is  plainly  our  duty  to  transmit  to 
posterity  accurate  measure  of  the  intensity  of  the  sun's  radiation  as  it 
is  now,  so  that  they  will  be  in  a  better  position  in  this  branch  of  science 
than  we  find  ourselves.  We  cannot  expect  to  know  the  intensity  of  the 
sun's  radiation  as  it  would  be  in  space  at  the  earth's  mean  distance  (that 


4-'  ASTRONOMY:  C.  G.  ABBOT 

is,  the  so-called  "solar  constant  of  radiation")  to  that  high  degree  of  ac- 
curacy which  we  are  accustomed  to  demand  in  many  branches  of  physics. 
If  we  determine  that  value  to  one*per;<:eHt,  it  is  all  we  may  reasonably  hope 
for,  because  the  intervention  of  the  earth's  atmosphere,  with  its  changing 
amounts  of  haze  and  cloudiness  will  always  interpose  to  the  investigator 
an  insuperable  obstacle  to  the  highest  degree  of  accuracy.  It  may  be 
that  in  the  future  apparatus  can  be  shot  up  by  means  of  rocket  devices 
to  go  outside  of  th6  atmosphere  altogether,  but  probably  the  sources  of 
error  of  automatic  apparatus  for  such  a  research  will  be  found  so  large 
that  no  greater  degree  of  accuracy  than  to  one  per  cent  will  be  possible 
even  with  them. 

The  spectrobolometric  investigations  of  this  subject  by  the  Astro- 
physical  Observatory  of  the  Smithsonian  Institution  which  have  been 
continued  for  the  last  seventeen  years  have  yielded  the  general  indication 
that  the  value  of  the  solar  constant  of  radiation  is  within  one  per  cent 
of  1.93  calories  per  square  centimeter  per-minute.  This  result  is  con- 
firmed by  experiments  made  with  sounding  balloons  by  the  same  observa- 
tory in  cooperation  with  the  U.  S.  Weather  Bureau  in  1914.  Automatic 
recording  pyrheliometers  were  sentvto  a  height  of  about  25  kilometers. 
It  was  found  that  the  intensity  of  the -radiation  there,  at  a  point  where 
24/25  of  the  atmosphere  was  beneath  the  observing  instrument,  fell  within 
the  range  of  solar  constant  values  as  obtained  by  spectrobolometric  work 
at  the  surface  of  the  Dearth.  The  actual  value  obtained  by  the  balloon 
work  was  1.84  calories  per -square  centimeter  per  minute.  Allowing  for 
the  obstruction  of  the  sun  rays  by  the  still  superincumbent  atmosphere 
a  reasonable  extrapolation  would  give -1.88 u calories.  This  differs  from 
1.93  calories  less  than  the  error  of  the  observations  and  is  well  within  the 
range  of  solar  constant;  values -obtained  at  different  days  from  surface 
observatories. 

Apparently  the ,  close  agreement  of  these  widely  different  independent 
methods  of  obtaining  the  Jn tensity  of  solar  radiation  outside  our  atmos- 
phere permits  us  to  say  authoritatively  that  the  mean  value  of  the  solar 
constant  is  at  any  rate  between  1.9  and  2.0  calories  per  square  centimeter 
per  minute  and  most  probably  not  greatly  different  from  1.93.  A  few 
men  cling  to  the  view  that  much  higher  values  should  be  set  for  it — values 
between  3  and  4  calories  per  square  centimeter  per  minute,  but  their  argu- 
ments carry  little  weight,  as  it  seems  to  me,  and  those  best  qualified  to 
know  are  agreed  that  the  values  given  above  -have  a  strong  foundation. 

The  methods  of  observing  the  solar  constant  of  radiation  by  means  of 
the  spectrobolometer  have  been  carefully  worked  out  and  the  whole  sub- 
ject has  been  published  with  satisfactory  fullness  and  is  in  shape  to  be 
transmitted  to  the  scientific  men  of  the  future  in  order  to  enable  them 
to  reproduce  the  measurements  or  to  compare  their  own  measurements 
with  the  results  now  obtained. 


ASTRONOMY:  C.  G.  ABBOT  "5 

2.  The  Sun's  Variability. — Passing  now  to  the  second  branch  of  the 
subject,  the  investigations  of  the  Smithsonian  Astrophysical  Observatory 
conducted  at  Washington,  Mt.  Wilson,  Mt.  Whitney,  Bassour  (Algeria), 
and  now  the  investigations  supported  by  the  Smithsonian  Institution 
from  its  private  funds  in  North  Carolina  and  Chile  have  all  united  in  giving 
the  impression  that  the  solar  radiation  is  not  constant,  but  varies  from 
day  to  day  through  a  range  of  certainly  five  and  possibly  at  times  ten 
per  cent.  The  conclusion  that  the  sun  is  a  variable  star  is  confirmed 
in  several  ways  but  most  notably  by  the  results  of  measurements  made  by 
the  Smithsonian  Astrophysical  Observatory  at  Mt.  Wilson,  California, 
on  the  distribution  of  energy  along  the  diameter  of  the  solar  image.  These 
measurements  indicate,  as  was  well  known  before,  that  the  edge  of  the 
sun's  disc  is  less  bright  than  the  center,  and  that  the  contrast  of  brightness 
between  the  center  and  the  edge  varies  according  to  the  wave-length  of 
light,  being  greater  for  short  wave-lengths,  less  for  long. 

But  the  measurements  of  recent  years  have  shown  that  not  only  is  there 
a  variation  of  contrast  by  wave-length,  but  also  a  variation  of  contrast  with 
the  time.  The  contrast  in  each  wave-length  is  different  for  different 
days  of  observation  and,  on  the  average,  for  different  years  of  observa- 
tion. The  changes  of  contrast  have  been  compared  with  the  changes  of 
total  radiation  of  the  sun  determined  by  the  aid  of  the  pyrheliometer  and 
spectrobolometer,  and  it  is  found  that  there  is  a  moderate  degree  of  corre- 
lation between  them.  The  correlation  is  of  two  kinds.  For  variations 
of  long  periods  of  years,  high  values  of  the  solar  constant  are  found  associ- 
ated with  high  values  of  contrast  between  the  center  and  edge  of  the  sun. 
On  the  contrary,  for  the  short  period  variations  of  the  solar  radiation,  oc- 
cupying a  few  days,  weeks,  or  months,  it  is  found  that  high  values  of  the 
solar  radiation  are  associated  with  diminished  values  of  the  solar  contrast. 

The  cause  of  this  two-fold  variation  is  reasonably  explained.  When 
the  sun  grows  hotter  and  thus  increases  its  output  of  radiation  along  with 
increased  solar  activity,  as  indicated  by  sun  spots,  prominences,  and  other 
visible  solar  phenomena,  this  would  tend  to  cause  a  greater  degree  of  con- 
trast. For  since  if  the  solar  temperature  were  zero  there  would  be  zero 
contrast,  the  higher  the  temperature  the  higher  the  contrast.  But  the 
sun  is  probably  entirely  gaseous,  and  certainly  its  outer  layers  are  so,  and 
these  may  become  more  turbid  at  times,  just  as  the  earth's  atmosphere 
becomes  more  hazy  at  some  times  than  at  others.  Accompanying  in- 
creased turbidity  of  the  solar  atmosphere  there  would  be  found  a  diminished 
value  of  the  solar  constant  of  radiation.  But  since  the  path  of  the  solar 
ray  is  oblique  in  the  solar  atmosphere  near  the  edge  of  the  sun,  the  path 
is  longer  there  and  the  effects  of  the  turbidity  would  be  greater  at  the  edges 
rather  than  at  the  center.  Thus  with  the  increase  of  turbidity  the  contrast 
of  brightness  would  increase  accompanying  a  diminished  value  of  the 


6  ASTRONOMY:  C.  G.  ABBOT 

solar  constant  of  radiation.     In  this  way  it  appears  that  the  two-fold 
variations  of  the  sun  which  have  been  found  may  be  reasonably  explained. 

Investigations  Required 

1.  Solar  Radiation  Stations. — We  now  pass  to  the  applications  of  this 
discovery  of  the  variation  of  the  sun.  What  effects  does  it  have  upon  the 
climate  of  the  earth  and  upon  other  terrestrial  phenomena?  In  order  to 
cover  satisfactorily  this  interesting  and  utilitarian  branch  of  the  investi- 
gation it  is  necessary  to  have  good  solar  observations  daily.  Unfortunately 
there  is  no  region  in  the  world  where  the  sky  is  so  free  from  cloudiness 
that  a  single  observatory  can  obtain  daily  measurements  of  the  intensity 
of  solar  radiation  as  it  is  outside  the  earth's  atmosphere.  The  spectro- 
bolometric  investigation  of  the  solar  constant  of  radiation  requires  that 
the  observations  shall  extend  over  several  hours  from  the  time  when  the 
sun's  altitude  is  small  to  the  time  when  it  is  large,  so  as  to  determine  for 
many  wave-lengths  of  the  spectrum  what  increase  of  intensity  corresponds 
to  the  diminution  of  the  length  of  the  path  of  the  solar  rays  within  the 
earth's  atmosphere.  In  order  to  prevent  error  in  the  determination  of 
the  transparency  of  the  earth's  atmosphere,  the  sky  must  be  uniformly 
clear  during  the  interval  just  mentioned. 

This  is  a  condition  very  hard  to  fulfil.  All  stations  in  the  world  fulfil 
it  well  enough  for  certain  days  of  the  year,  some  stations  for  very  many 
days  of  the  year,  but  none  for  all.  The  experience  of  the  Smithsonian 
Astrophysical  Observatory  in  these  matters  indicates  that  probably  there 
is  no  region  in  the  world  where  more  than  250  satisfactory  days  in  the  year 
can  be  confidently  expected.  Accordingly,  it  is  highly  desirable  if  these 
solar  radiation  changes  have  notable  effects  upon  the  earth's  temperature 
and  other  meteorological  and  terrestrial  phenomena,  that  several  observa- 
tories shall  join  in  making  the  measurements  of  the  solar  constant  of  radia- 
tion from  day  to  day.  On  every  day  at  least  two  values  should  be  ob- 
tained strictly  comparable  with  one  another  in  their  method  of  observa- 
tion and  with  a  probable  accuracy  as  far  as  experimental  work  is  concerned 
to  the  order  of  one  per  cent.  The  mean  of  two,  or  still  better,  of  three  or 
four  such  values,  would  give  a  probable  accuracy  of  the  final  result  to 
better  than  one  per  cent,  and  if  a  series  of  such  observations  extending  over 
every  day  in  the  year  for  a  course  of  several  years  were  available,  it  would 
probably  be  possible  to  determine  very  well  the  effect  of  the  changes  of 
solar  radiation  on  the  terrestrial  climate. 

In  order  to  carry  on  the  work  in  this  way,  a  considerable  income  is  re- 
quired. The  experience  of  the  Smithsonian  Institution  in  its  several  ex- 
peditions to  Africa  and  to  South  America,  and  to  the  western  part  of 
North  America,  indicates  that  an  income  of  between  five  and  ten  thousand 
dollars,  preferably  as  much  as  eight  thousand  dollars  per  year,  for  each 
station  would  be  requisite.  If  four  stations  should  be  considered  compe- 

r 


ASTRONOMY:  C.  G.  ABBOT  7 

tent  to  carry  out  the  problem,  it  would,  therefore,  require  a  yearly  income 
of  from  thirty  to  forty  thousand  dollars  for  the  whole  series.  It  would  be 
preferable  that  the  work  should  be  under  a  central  direction  so  that  the 
various  measurements  might  be  strictly  comparable  and  homogeneous. 
If  the  Smithsonian  Institution,  with  its  long  experience  in  these  matters, 
had  available  for  this  purpose  an  income  of  the  amount  specified,  that 
would  offer  a  good  way  of  dealing  with  the  problem. 

On  the  other  hand,  it  may  be  that  support  for  the  project  would  be 
more  easily  obtained  if  the  several  governments  of  the  regions  where  the 
observatories  for  this  purpose  might  be  favorably  located  should  be  ap- 
proached and  asked  to  take  over  the  work.  The  establishment  of  such  new 
observatories  seems  not  to  be  urgent  for  another  year,  until  the  results 
obtained  in  Chile  by  the  Smithsonian  Expedition  shall  have  been  com- 
pared with  the  climate  of  Argentina,  as  is  now  being  done  by  Mr.  Helm 
Clayton,  so  as  to  see  if  there  is  a  well-justified  expectation  that  solar  ob- 
servations of  this  kind  will  be  useful  in  meteorology. 

2.  Autographic  Cloud  Observers. — Looking  to  the  possible  establishment 
of  such  solar  constant  observing  stations,  the  most  important  preliminary 
investigation  relates  to  the  choice  of  the  most  favorable  situations.  Ex- 
perience hitherto  has  shown  that  the  reported  results  of  observations  of 
cloudiness  by  meteorological  services  do  not  give  a  sound  index  of  the 
availability  of  situations  for  this  exacting  work.  In  all  of  our  experi- 
ments we  have  invariably  been  disappointed  in  not  observing  as  many 
days  as  the  reports  of  the  cloud  observations  have  led  us  to  expect.  Cer- 
tain portions  of  the  earth,  it  is  true,  have  the  reputation  of  being  cloud- 
less, among  them  the  Chilean  Desert,  the  Central  and  Western  Provinces 
of  Australia,  some  parts  of  South  Africa,  Upper  Egypt,  the  Desert  of 
Sahara,  the  Desert  of  Arabia,  regions  in  British  India,  and  possibly  others. 
What  would  facilitate  the  establishment  of  solar  observing  stations  more 
than  anything  else,  in  case  they  should  seem  desirable,  would  be  to  have 
a  long  series  of  cloud  observations  obtained  by  strictly  automatic  processes, 
independent  of  the  judgment  of 'Observers,  for  a  considerable  number  of  the 
most  promising  regions,  such  as  those  I  have  mentioned.  Such  observa- 
tions should  cover  the  whole  period  when  the  sun  is  above  the  horizon. 

Dr.  Humphreys  has  proposed  to  me  an  instrument  for  such  a  prelim- 
inary investigation  of  cloudiness.  It  is  based  on  the  principle  of  photo- 
graphing a  reflecting  ball  from  above.  The  sensitive  surface  would  be  that 
of  a  moving  film  or  plate  which  would  be  exposed  for  an  instant  once  in  a 
quarter  or  half  an  hour,  as  would  be  most  convenient,  and  would,  there- 
fore, give  a  series  of  photographs  of  all  the  clouds  which  occurred  in  the 
sky  at  the  instants  of  observation.  If  this  or  some  other  suitable  automatic 
device  for  photographing  the  whole  sky  could  be  prepared  and  employed 
by  the  meteorological  services  of  the  world  which  operate  in  the  most 
cloudless  regions,  the  work  of  establishing  solar  constant  stations,  if 


8  ASTRONOMY:  C.  G.  ABBOT 

these  should  be  needed,  would  be  materially  advanced.  Otherwise,  there 
is  no  method  except  to  go  to  a  region  that  is  recommended  and  be  dis- 
appointed and  sorry  that  the  region  was  chosen  but  without  knowing 
where  else  to  go,  just  as  has  occurred  before.  This  is  a  disheartening, 
costly  and  time-consuming  process,  and  might,  perhaps,  be  avoided  in 
the  way  above  suggested. 

3.  Pyrheliometric  Observations. — While  the  measurement  of  the  solar 
radiation  by  means  of  spectrobolometric  work  is  the  only  thorough  way 
available  in  which  to  determine  the  variation  of  the  sun,  situated  as  the 
observer  must  necessarily  be  beneath  an  atmospheric  sea  full  of  haze  and 
other  obstructing  materials,  yet  it  is  highly  important  to  have  measure- 
ments of  the  radiation  of  the  sun  made  at  the  earth's  surface  in  many 
parts  of  the  world.     A  series  of  measurements  of  this  kind  is  being  con- 
ducted by  the  U.  S.  Weather  Bureau  at  its  stations  in  Washington,  Madison, 
Lincoln,  and  Santa  Fe.     This  work,  in  charge  of  Professor  Kimball,  has 
gone  on  now  for  about  ten  years,  and  is  being  published  in  excellent  form 
in  the  Monthly  Weather  Review  of  the  U.  S.  Weather  Bureau.     Observa- 
tions were  also  undertaken  in  Egypt  and  a  report  was  published  several 
years  ago  by  Dr.  Shaw.     Observations  were  carried  on  for  a  number  of 
years  by  the  Harvard  College  Observatory  at  Arequipa,  in  Peru,  but  about 
a  year  ago  the  instrument  was  accidentally  broken.     The  Arequipa  series 
was  then  discontinued,  partly  because  the  Smithsonian  Institution  has 
its  spectrobolometric  and  pyrheliometric  station  in   Chile  not  far  from 
Arequipa.     Some  measurements  are  being  made  also  in  Argentina,  Brazil 
and  elsewhere.     There  are  also  pyrheliometric  instruments  at  the  Island 
of  Teneriffe  which  were  in  use  before  the  war,  but  whether  the  measure- 
ments are  being  conducted  there  regularly  now  I  am  not  aware.     Dr. 
Dorno,  of  Davos,  Switzerland,  has  made  such  measurements  with  many 
others  of  related  subjects  for  a  long  time,  and  has  just  published  a  great 
volume  of  valuable  results. 

It  would  be  very  desirable  if  work  of  this  kind  could  be  carried  on 
regularly  at  favorable  stations  employing  strictly  comparable  instru- 
ments. There  are  about  thirty  copies  of  the  Smithsonian  silver  disc 
pyrheliometer  distributed  to  a  number  of  different  parts  of  the  world 
but  as  stated  above  it  is  not  probable  that  during  the  period  of  war  they 
have  been  in  regular  use.  Many  of  them  are  in  very  cloudy  localities. 

4.  Distribution  of  Radiation  over  the  Solar  Disc. — As  stated  above,  it 
has  been  found  at  the  Smithsonian  observing  station  on  Mt.  Wilson, 
in  California,  that  the  distribution  of  radiation  along  the  diameter  of  the 
solar  disc  is  subject  to  variability  of  two  kinds — one  of  a  long  period  of 
years,  and  the  other  of  a  short  period  of  days,  weeks,  or  months,  and 
that  these  two  kinds  of  variation  are  fairly  closely  correlated  with  the 
variations  of  the  total  radiation  of  the  sun.     The  work  of  observing  the 
radiation  along  the  solar  diameter  requires  the  use  of  a  long  focus  telescope 


ASTRONOMY:  C.  G.  ABBOT  9 

and  a  spectrobolometer,  and  may  very  readily  be  carried  on  nearly 
simultaneously  with  measurements  of  the  solar  constant  of  radiation. 
Such  investigation  is  going  on  regularly  at  Mt.  Wilson  during  the  summer 
season  when  the  Smithsonian  observers  are  working  there.  Perhaps  it 
would  not  be  necessary  to  set  up  other  stations  since  the  main  object  of 
this  work  was  accomplished  when  it  was  shown  that  it  confirmed  the 
existence  of  the  solar  variability  shown  by  the  measurements  of  the  solar 
radiation  itself. 

The  method  of  observing  the  distribution  of  brightness  along  the  solar 
diameter  which  we  have  adopted  is  to  stop  the  clock-work  of  the  coelostat 
which  reflects  the  solar  beam  into  the  long  focus  telescope,  so  that  the 
solar  image  drifts  across  the  slit  of  the  spectrobolometer  in  an  east  and 
west  fashion.  The  question  was,  therefore,  open  for  some  time  whether 
the  distribution  at  right  angles  to  this,  that  is,  along  the  north  and  south 
diameter  of  the  sun,  would  be  substantially  the  same.  In  1918,  Mr. 
Aldrich  investigated  this  matter  by  the  aid  of  special  mechanism  associated 
with  the  coelostat  and  was  able  to  show  that  if  any  difference  in  distribu- 
tion exists  between  the  north  and  south  and  the  east  and  west  diameter 
of  the  solar  image,  respectively,  it  is  too  small  to  recognize  by  this  method 
and,  indeed,  is  less  than  one  per  cent. 

INVESTIGATIONS  OF  THE  RELATIONS  OF  TERRESTRIAL  CLIMATE  TO  RADIATION 

Survey  of  the  Present  State  of  the  Field 

1.  Kinds  of  Radiation  Concerned. — The  temperature  of  the  world  is 
maintained  by  the  balance  between  the  income  of  solar  radiation  and 
the  outgo  to  space  of  terrestrial  radiation.     The  former  is  principally 
included  between  the  wave-lengths  0.3  and  3.0  microns.     The  latter  is 
principally  included  between  the  wave-lengths  5.0  and  50  microns.     Sub- 
stances which  are  transparent  for  the  one  may  be  opaque  for  the  other, 
though  in  these  long  ranges  of  wave-lengths  substances  may  pass  through 
several  minima  or  maxima  of  opacity  and  transmissibility.     Apart  from 
cloudiness,  which  is  the  principal  factor,  the  radiation  of  the  sun  is  princi- 
pally hindered  in  its  passage  to  the  surface  of  the  earth  by  the  water 
vapor  of  the  terrestrial  atmosphere  and  by  the  scattering  of  the  mole- 
cules of  the  air  and  by  the  dust  which  the  air  carries.     The  losses  of  the 
direct  solar  beam  by  scattering  from  air  molecules  and  dust  are  largely 
compensated  for  by  the  indirect  solar  radiation  received  from  the  whole 
hemisphere  of  the  sky.     On  the  other  hand  the  rays  lost  from  the  direct 
beam  by  absorption  in  the  water  vapor  of  the  earth's  atmosphere,  while 
they  go  to  warm  the  atmosphere  itself,  are  very  differently  applied  to  * 
maintain  the  heat  of  the  earth  as  a  planet  than  they  would  be  if  they  reached 
the  earth's  surface. 

2.  Absorbents  of  Terrestrial  Radiation. — Apart  from  cloudiness,  which 
is  the  principal  hindering  factor,  the  outgoing  radiation  of  the  earth's 


10  ASTRONOMY:  C.  G.  ABBOT 

surface  to  space  is  hindered  mainly  by  water  vapor,  ozone,  and  carbon 
dioxide.  Of  these  the  principal  obstruction  is  water  vapor  which  absorbs 
powerfully  over  great  ranges  of  spectrum.  The  other  two  absorbents 
are  each  confined  in  their  absorbing  regions  to  comparatively  narrow  ranges 
of  spectrum,  but  the  ozone  absorption  band,  at  about  10  microns,  occurs 
in  a  region  where  water  vapor  absorbs  scarcely  anything  while  the  carbon 
dioxide  absorption  band  at  about  14  microns  occurs  in  a  region  where  water 
vapor  is  also  powerfully  absorbing.  The  atmospheric  proportion  of  carbon 
dioxide  is  sensibly  constant,  while  water  vapor  and  ozone  are  variable. 
Accordingly,  while  water  vapor  is  certainly  the  most  important  of  the  three, 
probably  ozone,  although  much  less  plentiful  in  the  atmosphere,  and  cer- 
tainly not  more  powerful  as  an  absorber  for  the  spectrum  of  a  perfect 
radiator  than  carbon  dioxide,  is  yet  entitled  to  be  regarded  as  second  in 
importance  on  acount  of  this  peculiar  posture  of  affairs. 

3-  Stiidies  of  Terrestrial  Radiation  and  Its  Atmospheric  Absorption. — 
The  study  of  these  absorbing  elements  and  their  influence  upon  the  spec- 
trum has  been  carried  on  to  a  considerable  extent  by  several  investigators, 
notably  by  Paschen,  Rubens  and  his  collaborators,  Miss  Eva  von  Bahr, 
and  lately  by  Mr.  Fowle  of  the  Astrophysical  Observatory  of  the  Smith- 
sonian Institution.  Mr.  Fowle's  researches2  comprised  the  examination 
of  the  spectrum  of  the  Nernst  Glower  up  to  a  wave-length  of  about  17 
microns,  particularly  as  it  is  affected  by  the  absorption  occurring  in  long 
columns  of  atmospheric  air  containing  known  amounts  of  humidity  and 
carbon  dioxide.  The  experiments  deal  with  quantities  of  from  0.003 
to  3.0  centimeters  of  liquid  water  disposed  in  the  atmosphere  as  water 
vapor.  Owing  to  the  lack  of  a  suitable  transparent  refracting  medium 
for  optical  purposes  beyond  the  wave-length  17  microns,  the  important 
region  of  the  terrestrial  spectrum  extending  from  17  to  50  microns  could 
not  be  investigated  by  Mr.  Fowle.  Somewhat  later,  Mr.  Aldrich  of  the 
Astrophysical  Observatory  of  the  Smithsonian  Institution  made  a  long 
series  of  experiments  (as  yet  unpublished)  with  crystals  both  natural  and 
artificial  and  with  other  available  substances,  endeavoring  to  discover 
one  which  would  be  suitable  for  making  a  prism  transparent  to  these 
rays.  The  only  substance  found  which  seemed  to  be  notably  superior 
to  rock  salt  is  potassium  iodide,  and  this  material  has  hitherto  been  avail- 
able only  in  very  minute  crystals.  Promising  experiments  made  at  the 
General  Electric  Company  indicate  that  large  crystals  of  potassium 
iodide  for  prisms  may  soon  be  available.  If  so,  it  will  be  possible  to  carry 
on  this  valuable  spectrum  investigation  over  a  region  hitherto  very  little 
explored.  The  dependence  of  terrestrial  radiation  on  water  vapor  has 
been  investigated  to  a  considerable  extent  in  another  way  by  Dr.  A.  K. 
Angstrom,  who  made  observations  in  Algeria,  Southern  California,  and 
lately  in  Sweden  with  the  pyrgeometer,  a  nocturnal  radiation  measuring 
instrument  invented  by  K.  Angstrom.  Dr.  Angstrom's  measurements 


ASTRONOMY:  C.  G.  ABBOTT  11 

were  conducted  during  several  months  at  a  considerable  number  of  sta- 
tions varying  in  altitude  from  near  sea  level  to  that  of  Mt.  Whitney, 
4,400  meters,  and  in  dryness  from  desert  conditions  to  those  of  consider- 
able humidity.  At  the  same  time  measurements  were  made  of  the  hu- 
midity prevailing  in  the  air,  partly  at  the  surface  by  means  of  psychrome- 
ters  and  partly  in  the  free  air  by  means  of  kites  and  sounding  balloons 
under  the  management  of  the  United  States  Weather  Bureau.  This 
valuable  investigation  was  supported  by  the  Hodgkins  Fund  of  the  Smith- 
sonian Institution,  and  was  published  as  Vol.  65,  No.  3,  of  the  Smith- 
sonian Miscellaneous  Collections. 

Investigations  Required 

1.  Ozone  and  Radiation. — Very  little  is  known  concerning  the  influence 
of  ozone  on  the  earth's  radiation.     It  is  certain  that  a  conspicuous  band  of 
this  substance  occurs  at  the  wave-length  of  about  10  microns,  where  water 
vapor  is  almost  completely  transparent.     It  is  a  little  hard  to  see  how  the 
investigation  of  the  influence  of  ozone  can  be  best  undertaken.     The 
region  of  10  microns  is  very  difficult  to  work  with  in  the  solar  spectrum. 
All  terrestrial  surroundings  give  out  waves  of  this  wave-length.     It  is 
as  if  the  investigator  of  the  visible  spectrum  were  troubled  with  stray 
light  from  every  direction  upon  his  photographic  plates  or  other  means 
of  observation.     Furthermore,  and  still  more  serious,  the  more  intense 
rays  of  the  solar  spectrum,  coming  from  regions  of  much  shorter  wave- 
length, are  continually  encountered  as  stray  light  in  the  spectrum,  even 
hundreds  of  fold  greater  in  intensity  than  the  rays  of  the  spectrum  at 
the  region  in  question.     Rock  salt  must  be  employed  for  the  optical  train 
and  its  hygroscopic  character  is  a  serious  difficulty.     Finally  the  measure- 
ments ought  to  be  quantitative  in  terms  of  energy.     Something  can  be 
done  to  avoid  stray  light  by  reflecting  the  solar  beam  by  means  of  several 
silvered  ground  glass  mirrors.     In  this  way  the  shorter  wave-lengths  are 
scattered,  while  the  longer  wave-lengths  desired  are  reflected  by   virtue 
of  the  greatness  of   their  wave-length,  compared  with  the  roughness  of 
the  mirrors.     Yet  the  prospects  of  getting  good  bolographic  representa- 
tions of  this  region  of  spectrum  day  after  day  are  not  good.     Perhaps 
the  quantity  of  ozone  existing  in  the  air  could  be  determined  daily  by 
measurements  made  in  the  ultra-violet  spectrum  by  the  aid  of  photography 
and  these  results  could  be  compared  with  measurements  of  the  nocturnal 
radiation  made  after  the  method  of  Angstrom.     Allowance  being  made  for 
variations  of  temperature  and  water  vapor,  the  influence  of  variations 
of  the  ozone  might  then  appear.     At  all  events,  determination  of  the  effects 
of  ozone  on  the  terrestrial  radiation  is  a  desirable  investigation  though  so 
extremely  difficult. 

2.  Cloud  Observations. — The  effect  of  clouds  on  terrestrial  temperature 
is  extremely  important.     Recent  measurements  by  Mr.  Aldrich  of  the 


12  ASTRONOMY:  C.  G.  ABBOT 

Smithsonian  Institution,  made  from  a  military  balloon,  show  that  a  smooth 
layer  of  clouds  reflects  about  78%  of  the  total  direct  and  indirect  solar 
radiation  incident  upon  it  from  the  sun  and  sky.  Investigations  of  the 
quantity  and  distribution  of  the  cloudiness  of  the  surface  of  the  earth 
have  been  published  by  Tisserenc  du  Bort,  and  Arrhenius.  They  de- 
pend, however,  on  very  imperfect  material.  Cloud  observations  are 
much  more  fragmentary  than  temperature  observations,  and  besides 
are  not  based  upon  standardized  instruments  in  general,  even  if  there  be 
such,  but  are  based  on  observers'  estimates  of  the  proportion  of  the  sky 
obscured.  The  matter  is  so  fundamental  to  the  proper  understanding 
of  the  temperature  of  the  world  and  its  dependence  on  the  balance  of 
radiations  of  the  sun  and  earth  that  it  warrants  careful  attention.  Some 
simple  autographic  means,  theoretically  satisfactory  and  instrumentally 
accurate,  ought  to  be  provided  for  estimating  cloudiness,  and  this  should 
be  installed  with  the  least  possible  delay  at  all  first-rate  meteorological 
stations  in  the  world,  and  indeed  as  extensively  as  possible.  The  instru- 
ment should  not  be  merely  applicable  to  land  conditions,  but  ought  to 
be  available  for  ships.  It  is  hardly  possible  to  overestimate  the  importance 
of  this  neglected  branch  of  meteorology. 

3.  Standard  Instruments  for  Nocturnal  Radiation. — As  mentioned 
above,  Dr.  Angstrom  has  employed  the  pyrgeometer  for  an  extensive  and 
valuable  research  on  nocturnal  radiation  and  the  determination  of  at- 
mospheric re-radiation  at  different  temperatures  and  under  various  other 
meteorological  conditions.  There  has  also  been  devised  at  the  Smithsonian 
Institution,  by  Messrs.  Abbot  and  Aldrich,  an  instrument  called  the 
pyranometer,  which  is  applicable  for  the  same  purpose.  Neither  of  these 
instruments,  however,  is  thoroughly  satisfactory,  for  this  reason:  They 
both  present  blackened  flat  surfaces  which  radiate  towards  the  atmosphere 
and  space,  and  it  is  assumed  in  the  theory  of  the  instrument  that  the 
absorbing  and  radiating  properties  of  the  blackening  upon  these  strips  is 
known.  Experiments,  however,  seem  to  show  that  the  materials  used 
for  the  blackening  purposes  become  less  and  less  satisfactory  for  the  longer 
wave-lengths,  so  that  for  rays  beyond  15  microns  there  is  a  strong  chance 
that  the  instruments  are  very  imperfect.  In  order  to  really  cover  the 
objection  it  is  necessary  to  produce  a  nocturnal  radiation  instrument  which 
employs  the  well-known  hollow  receiver  principle  of  the  "black  body." 
This  is  a  matter  of  some  difficulty  because  the  intensity  of  the  radiation 
to  be  considered  is  small,  and  it  must  be  emitted  and  received  at  all  angles 
within  a  hemisphere.  There  are  very  formidable  difficulties  in  the  way 
of  a  satisfactory  solution  of  this  requirement,  but  in  order  to  give  really 
definite  information  as  to  the  nocturnal  radiation  of  the  earth  and  at- 
mosphere they  must  be  overcome.  Associated  with  this  problem  is  a 
definitive  solution  of  the  question  of  the  constant  of  the  fourth  power 
radiation  formula  of  Stefan.  The  measurements  hitherto  made  by  physi- 


ASTRONOMY:  C.  G.  ABBOT  13 

cists  on  this  constant,  usually  called  <r,  are  in  considerable  disaccord.  The 
margin  of  possible  error  is,  perhaps,  not  very  large,  but  it  cannot  be  allowed 
as  certain  that  we  know  this  constant  within  2%. 

4.  Statistical  Studies. — As  already  stated,  the  sun  is  known  to  be  a 
variable  star.  The  effect  of  its  variations  on  the  terrestrial  climate 
has  lately  been  investigated  to  a  considerable  extent  by  Mr.  H.  H.  Clayton2 
of  Buenos  Aires.  Investigations  of  a  similar  kind  have  been  made  also 
by  Dr.  Arctowski  of  New  York  and  by  Dr.  Helland-Hansen  and  Dr. 
Nansen3  of  Norway.  These  investigators  have  all  reached  the  conclu- 
sion that  the  variations  of  the  sun  reported  by  the  Smithsonian  observers 
at  Mt.  Wilson  affect  terrestrial  temperatures  to  a  considerable  extent  and 
also  induce  variations  of  terrestrial  atmospheric  pressure.  Mr.  Clayton's 
investigations  have  led  him  to  the  conclusion  that  the  tropical  and  the 
polar  regions  of  the  earth  are  made  warmer  by  temporary  increases  of 
solar  radiation,  while  the  temperate  regions  both  north  and  south  of  the 
equator  are  made  cooler  by  the  same  increase  of  radiation.  Dr.  Nansen, 
however,  is  of  the  opinion  that  when  more  thorough  studies  of  the  matter 
have  been  made,  it  will  be  found  that  the  effects  are  not  thus  zonal,  but 
rather  are  associated  with  the  great  action  centers  of  atmospheric  circula- 
tion. Dr.  Nansen's  view  is  that  since  a  considerable  portion  of  the  sun's 
radiation  is  absorbed  by  the  clouds,  dust  and  water  vapor  in  the  atmos- 
phere, a  change  in  the  amount  of  solar  radiation  may  produce  notable 
changes  in  the  atmospheric  circulation,  and  these,  by  altering  the  direction 
of  the  prevailing  winds,  may  alter  indirectly  the  temperature  of  meteoro- 
logical stations.  Hitherto  the  amount  of  investigation  of  the  variation 
of  the  sun  has  been  inadequate  to  give  very  satisfactory  data  to  work  with 
in  determining  these  correlations.  The  present  expedition  of  the  Smith- 
sonian Institution  to  Chile,  it  is  hoped,  will  be  continued  for  several 
years  and  will  enable  meteorologists  to  trace  this  relation  much  more 
thoroughly.  Mr.  Clayton,  from  the  studies  he  has  made  and  is  making, 
is  very  sanguine  about  the  value  of  solar  radiation  observations  such  as 
are  being  made  by  the  Smithsonian  observers. 

The  accompanying  illustration,  prepared  from  Clayton's  studies,  shows 
the  remarkable  courses  of  temperature  departures  at  Buenos  Aires,  fol- 
lowing high,  intermediate  and  low  values  of  the  solar  constant  of  radiation. 

A  very  great  deal  of  additional  statistical  inquiry  is  already  possible 
in  view  of  the  series  of  solar  constant  observations  reported  by  the  Smith- 
sonian observers  on  Mt.  Wilson.  There  is  now  a  much  more  continuous 
series  of  results  available  extending  from  July  27,  1918,  as  obtained  at 
Calama,  Chile.  Meteorologists  should  be  in  a  position  to  study  these 
observations  in  connection  with  the  temperature,  pressure,  cloudiness 
and  wind  at  a  very  great  number  of  stations  distributed  as  well  as  possible 
over  the  earth's  surface.  It  is  only  by  such  laborious  investigation 


14 


ASTRONOMY:  C.  G.  ABBOT 


that  a  sound  basis  can  be  laid  for  the  improvements  in  forecasting  which 
Mr.  Clayton  thinks  are  now  possible. 

If  it  shall  appear  from  such  statistical  investigations  that  the  varia- 
tions of  the  sun  are  really  important  for  forecasting  purposes,  and  if 


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it  shall  prove  possible  to  establish  three  or  four  additional  solar  radia- 
tion stations,  widely  scattered  in  the  most  cloudless  regions  in  the  world, 
then  we  may  contemplate  the  possibility  that  from  each  such  solar  radia- 
tion station  the  results  of  each  day  might  be  sent  out  by  wireless  telegraphy, 
to  be  employed  by  the  meteorological  stations  of  the  different  countries 
for  forecasting  purposes.  Experience  in  Calama,  Chile,  indicates  that 


ASTRONOMY:  C.  G.  ABBOT  15 

with  a  staff  of  three  industrious  and  energetic  persons  it  is  possible  to 
compute  and  send  out  the  result  of  morning  observations  of  a  given  day 
before  night-fall.  A  brief  empirical  method  for  the  determination  of  solar 
radiation  has  lately  been  devised  at  Calama  and  seems  satisfactory,  so 
that  it  is  possible  that  the  result  of  a  morning  of  observation  may  be  tele- 
graphed to  the  world  by  noon. 

1  This  is  issued  as  the  second  of  a  series  of  research  surveys  prepared  under  the  auspices 
of  the  National  Research  Council.     Reprinted  in  Reprint  and  Circular  Series  of  the 
National  Research  Council,  Number  7. 

2  Smithsonian  Misc.  Coll.  Washington,  68,  No.  3,  1917. 

1  Now  in  course  of  publication  by  the  Smithsonian  Institution. 


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